simple: enforce early return in loops

Early returns help to reduce nesting levels. I wonder how much effort is it to implement with staticcheck.io. Examples are:

unwanted:
```go
for _, v := range x {
    if condition {
        // do stuff
    }
}
```

wanted:
```go
for _, v := range x {
    if negatedCondition {
        continue
    }
    // do stuff
}
```